Aerodynamic Roughness Research Articles

Investigating microclimate effects in an oasis-desert interaction zone

To investigate oasis-desert microclimate effects, we performed a series of numerical simulations in an idealized oasis-desert system based on an improved computational fluid dynamics (CFD) model for simulating atmospheric boundary layer flows, air temperature and humidity. Numerical simulations were designed based on the hydrometeorological observations obtained during the HiWATER-MUSOEXE (Heihe Watershed Allied Telemetry Experimental Research, Multi-Scale Observation Experiment on Evapotranspiration over heterogeneous land surfaces) campaign. The results are summarized as follows: (1) Oasis-desert interactions are significantly affected by background wind conditions. We observed the oasis-desert local circulation under calm background wind conditions and the oasis thermal internal boundary layer under low wind speed conditions induced by hydrothermal contracts. These interactions will disappear when the background wind speed is sufficiently high, and there is only an oasis dynamic internal boundary layer caused by the aerodynamic roughness length contrast. (2) Oasis-desert interactions lead to a series of microclimate effects, including the oasis cold-wet island effect, air humidity inversion effect within the surrounding desert and oasis wind shield effect, which are important for the stability and sustainability of the oases-desert ecosystem. (3) The hydrothermal conditions due to the difference between the oasis and desert, the vegetation fraction and distribution patterns impact the oasis-desert microclimate effects. The intensity of oasis-desert interactions increases with the land surface temperature (LST) difference in the oasis-desert. The oasis-desert interactions are gradually strengthened with the increase of the vegetation fraction within the oasis. Integrated ecological and economic benefits of the oasis, the oasis vegetation pattern, which includes the croplands and shelterbelts staggered within the oasis and the shelterbelts surrounding the outside, is beneficial to limiting the loss of water vapor and preventing sandstorms from the oasis. The findings of the current study improve the fundamental understanding of the microclimate and provide implications for maintaining the sustainability of oasis-desert ecosystems.

Threshold friction velocities influenced by standing crop residue in the inland Pacific Northwest, USA

AbstractCrop residue is an important factor affecting soil wind erosion. Few studies have tested the effects of crop residue on threshold friction velocities (U*t) which not only controls the initiation of wind erosion events but also the frequency and intensity of erosion events and thus the rate of land degradation. This study measured the threshold friction velocity (U*t) and aerodynamic roughness length (Z 0) in the presence of winter wheat (Triticum aestivum L.), canola (Brassica napus L.), and garbanzo (Cicer arietinum) standing residue arrays to identify the sheltering capability against wind erosion of these different vegetation patterns. The influence of crop residue density and lateral cover (total roughness frontal area to soil surface area) on U*t and Z 0 was tested in a wind‐tunnel. The results indicated U*t increased linearly with residue density for the three residue types and a similar trend was found for Z 0. However, U*t and Z 0 varied among crop residue types at the same residue density or frontal area index (λ). Based on the slope of regression analysis, U*t and Z 0 of garbanzo were 40 and 25% higher than winter wheat and 43 and 119% higher than canola. This study suggested that crop residue type is important in determining U*t and Z 0 when modeling wind erosion.

Effects of inflow turbulence intensity and turbine arrangements on the power generation efficiency of large wind farms

AbstractIn this study, we conduct a series of large‐eddy simulations (LESs) to study the impact of different incoming turbulent boundary layer flows over large wind farms, with a particular focus on the overall efficiency of electricity production and the evolution of the turbine wake structure. Five representative turbine placements in the large wind farm are considered, including an aligned layout and four staggered layouts with lateral or vertical offset arrangements. Four incoming flow conditions are used and arranged from the LESs of the ABL flow over homogeneous flat surfaces with four different aerodynamic roughness lengths (i.e., z0 = 0.5, 0.1, 0.01, and 0.0001 m), where the hub‐height turbulence intensity levels are about 11.1%, 8.9%, 6.8%, and 4.9%, respectively. The simulation results indicate that an enhancement in the inflow turbulence level can effectively increase the power generation efficiency in the large wind farms, with about 23.3% increment on the overall farm power production and up to about 32.0% increment on the downstream turbine power production. Under the same inflow condition, the change of the turbine‐array layouts can increase power outputs within the first 10 turbine rows, which has a maximum increment of about 26.5% under the inflow condition with low turbulence. By comparison, the increase of the inflow turbulence intensity facilitates faster wake recovery that raises the power generation efficiency of large wind farms than the adjustment of the turbine placing layouts.

Spatial and temporal variations in glacier aerodynamic surface roughness during the melting season, as estimated at the August-one ice cap, Qilian mountains, China

Abstract. The aerodynamic roughness of glacier surfaces is an important factor governing turbulent heat transfer. Previous studies rarely estimated spatial and temporal variation in aerodynamic surface roughness (z0) over a whole glacier and whole melting season. Such observations can do much to help us understand variation in z0 and thus variations in turbulent heat transfer. This study, at the August-one ice cap in the Qilian mountains, collected three-dimensional ice surface data at plot scale, using both automatic and manual close-range digital photogrammetry. Data were collected from sampling sites spanning the whole ice cap for the whole of the melting season. The automatic site collected daily photogrammetric measurements from July to September of 2018 for a plot near the center of the ice cap. During this time, snow cover gave way to ice and then returned to snow. z0 was estimated based on micro-topographic methods from automatic and manual photogrammetric data. Manual measurements were taken at sites from the terminals to the top of the ice cap; they showed that z0 was larger at the snow and ice transition zone than in areas that are fully snow or ice covered. This zone moved up the ice cap during the melting season. It is clear that persistent snowfall and rainfall both reduce z0. Using data from a meteorological station near the automatic photogrammetry site, we were able to calculate surface energy balances over the course of the melting season. We found that high or rising turbulent heat, as a component of surface energy balance, tended to produce a smooth ice surface and a smaller z0 and that low or decreasing turbulent heat tended to produce a rougher surface and larger z0.

Stable and convective boundary-layer flows in an urban array

In this paper non-neutral approaching flows were employed in a meteorological wind tunnel on a regular urban-like array of rectangular buildings. As far as stable stratification is concerned, results on the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length up to 80%. The roughness length and displacement height were also affected, with a reduction up to 27% for the former and an increment up to 5% for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratification cases, the friction velocity appears increased by both the effect of roughness and unstable stratification. The increased roughness causes a reduction in the surface stratification, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching flow. The effect on the aerodynamic roughness length and displacement height are specular to the SBL case, an increase up to 55% of the former and a reduction of the same amount for the latter.

Improving surface roughness lengths estimation using machine learning algorithms

Surface roughness lengths, including the aerodynamic roughness length (z0m) and the thermodynamic roughness length (z0h, represented by excess resistance kB−1), are crucial parameters in the accurate simulation of surface turbulent fluxes. However, due to insufficient knowledge in the physical mechanisms of surface roughness lengths, there exist considerable uncertainties in physically-based models. In this study, we attempt to overcome this issue by establishing the data-driven surface roughness lengths models, which are based on global observations from the FLUXNET2015 dataset and Moderate Resolution Imaging Spectroradiometer (MODIS). Four machine learning algorithms, including random forest (RF), single hidden layer artificial neural network (ANN), multilayer perceptron (MLP), deep belief network (DBN) are explored. A large number of data from 45 flux tower sites (as many as 44,662 daily z0m and 583,484 half-hour kB−1 observations) are utilized to train and test the data-driven models. Our results show that the data-driven models surprisingly achieve significantly improved estimation of surface roughness lengths and turbulent fluxes than physical models, which indicated the model inadequacy of physical models. RF-driven models achieve the best results. MLP and DBN-driven models of higher complexity are slightly superior to ANN-driven models but exhibit unstable performance. RF and ANN accurately reproduce the unimodal function relationship between leaf area index and z0m, thus demonstrating that the machine learning methods can extract physical rules from vast numbers of observations. In contrast, MLP and DBN fail to capture this relationship, possibly because of too complicated architecture. It implies that a suitable complexity of machine learning algorithm is critical to excavate true physical mechanism. To the best of our knowledge, this is the first study to demonstrate that machine learning technique can contribute to highly accurate estimations of surface turbulent fluxes by building data-driven surface roughness lengths models.

Desert Roughness Retrieval Using CYGNSS GNSS-R Data

The aim of this paper is to assess the potential use of data recorded by the Global Navigation Satellite System Reflectometry (GNSS-R) Cyclone Global Navigation Satellite System (CYGNSS) constellation to characterize desert surface roughness. The study is applied over the Sahara, the largest non-polar desert in the world. This is based on a spatio-temporal analysis of variations in Cyclone Global Navigation Satellite System (CYGNSS) data, expressed as changes in reflectivity (Γ). In general, the reflectivity of each type of land surface (reliefs, dunes, etc.) encountered at the studied site is found to have a high temporal stability. A grid of CYGNSS Γ measurements has been developed, at the relatively fine resolution of 0.03° × 0.03°, and the resulting map of average reflectivity, computed over a 2.5-year period, illustrates the potential of CYGNSS data for the characterization of the main types of desert land surface (dunes, reliefs, etc.). A discussion of the relationship between aerodynamic or geometric roughness and CYGNSS reflectivity is proposed. A high correlation is observed between these roughness parameters and reflectivity. The behaviors of the GNSS-R reflectivity and the Advanced Land Observing Satellite-2 (ALOS-2) Synthetic Aperture Radar (SAR) backscattering coefficient are compared and found to be strongly correlated. An aerodynamic roughness (Z0) map of the Sahara is proposed, using four distinct classes of terrain roughness.

Sheltering effect of a multiple-row Tamarix windbreak – a field study in Niatak, Iran

The Sistan region in southeastern Iran is one of the country's main dust storm areas. To reduce the frequency and intensity of dust storms using efficient windbreaks is critical. In a field study the sheltering effect of a multiple-row windbreak (14 rows of Tamarix aphylla) in the Niatak area was evaluated by obtaining the vertical and horizontal values of Rc(x, z) (wind reduction coefficient) upwind (x=− 100), within (x=100 and 256) and downwind (x=448 and 560) of the windbreak. A slight deflection in the vertical profiles of the sheltering effect at a height ofz/h= 0.2–0.8, a significant increase in the Rc(x, z) values was observed within the windbreak that demonstrated the roughness influence of vegetation. The sheltering effect increased within the windbreak consistent with the increasing aerodynamic roughness length (z0). The greatest Rc(x, z) values was observed at the beginning of windbreak (x= 100), where the largest values of z0 occurred. At downwind distances of x=448 and 560 the values of Rc(x, z) were respectively greater and lower than upwind values, revealing the sheltering effect of the windbreak in an area 50 h beyond the vegetation. Significant sheltered areas (50 h beyond and 1.8 h above the windbreak) and reduced wind speeds (50% at x=100) by the windbreak and the location of minimum wind speed at the beginning of the windbreak demonstrates that the efficiency of the windbreak is optimum. This is mainly attributed to the optimal porosity (39%) and structure of the windbreak (multiple-row design).

Influence of Atmospheric Stability on the Size Distribution of the Vertical Dust Flux Measured in Eroding Conditions Over a Flat Bare Sandy Field

AbstractIn spite of their importance for the modeling of the atmospheric cycle of mineral dust, measurements of the intensity and size distribution of the dust emission flux produced by wind erosion in natural conditions remain rare. During the WIND‐O‐V's (WIND erOsion in presence of sparse Vegetation) 2017 experiment, eight major erosion events having occurred on a sandy flat field of southern Tunisia were documented. Consistent with the small size (90 μm) of the erodible sand grains and the low aerodynamic roughness length (Z0 < 0.079 cm), the threshold for wind erosion was low (u*t = 22 cm/s). The classical gradient method was applied to assess the size‐resolved vertical dust flux, and the stability of the atmosphere quantified by the means of the Richardson number (Ri) as well as of its shear stress (u*) and thermal gradient (∂θ/∂z) components. The vertical dust flux increased with u* following a power law but the number size distribution of the dust flux was found to be significantly richer in submicron particles in thermally unstable than in stable periods. This challenges the usual assumption that, independently of their size, the particles smaller than 10 μm follow equally the movements of the air masses in which they are embedded and that the thermal stratification of the surface layer does not affect the size distribution of the surface flux when measured a few meters above the ground. Finally, we propose a simple empirical method for taking this influence of the thermal instability into account.

Glacial Aerodynamic Roughness Estimates: Uncertainty, Sensitivity, and Precision in Field Measurements

AbstractCalculation of the sensible and latent heat (turbulent) fluxes is required in order to close the surface energy budget of glaciers and model glacial melt. The aerodynamic roughness length, z0, is a key parameter in the bulk approach to calculating sensible heat flux; yet, z0 is commonly considered simply as a tuning parameter or generalized between surfaces and over time. Spatially and temporally distributed observations of z0 over ice are rare. Both direct (from wind towers and sonic anemometers) and indirect (from microtopographic surveys) measurements of z0 are subject to sensitivities and uncertainties that are often unstated or overlooked. In this study, we present a quantitative evaluation of aerodynamic profile‐based and microtopographic methods and their effect on z0 using data collected from Storglaciären and Sydöstra Kaskasatjäkkaglaciären, Tarfala Valley, Arctic Sweden. Aggressive data filters discard most of the wind tower data but still produce realistic z0 values of 1.9 mm and 2 mm. Despite uncertainty introduced by scale and resolution dependence, microtopographic methods produced estimates of z0 comparable to wind tower values and those found on similar surfaces. We conclude that (1) in the absence of direct turbulent flux measurements from sonic anemometers, the profile and microtopographic methods provide realistic z0 values, (2) both 2D and 3D microtopographic methods are dependent on scale, resolution, and the chosen detrending method, and (3) careful calibration of these parameters could enable glacier‐wide investigations of z0 from remotely sensed data, including those increasingly available from satellite platforms.

Investigating the impacts of driving factors on urban heat islands in southern China from 2003 to 2015

Southern China is susceptible to urban heat island (UHI) effect due to the rapid urbanization and subtropical climate. However, comprehensively assessing long-term UHI and its driving factors across southern China remains elusive. In this study, we investigate the UHI effect and its relationship with several critical factors in the urban agglomerations of southern China from 2003 to 2015 using remotely sensed derived products and land surface Noah model outputs. Results indicate that (1) the UHI intensity (UHII) in southern China is 1.49 K with an increasing trend of 0.011 ± 0.004 K per year. This relates to the land cover changes caused by the urbanization, which explains ∼40% of UHII trend variance; (2) Resided in the subtropical maritime climate, UHII across southern China is closely related to two climate factors: solar radiation and precipitation. Specifically, the adverse impact of precipitation on UHII is significant across southern China, where sufficient rainfall can modify soil moisture condition and improve the thermal regulation of watershed; (3) Diurnal temperature range and surface net radiation that describes surface property cannot fully explain UHII across southern China. Nevertheless, variations of sensible flux and latent flux correlate better with UHII. The sensible flux (associated with aerodynamic roughness) is responsible for 32% of UHII while the latent flux (attributed to evaporation cooling) is responsible for 13% of UHII. This result underscores the substantial contribution of sensible flux on UHI. In addition, our study suggests the effect of anthropogenic intervention may could explain the inconsistent performances of heat fluxes in responding to UHI. Our findings highlight the significance of land-atmosphere controlling factors on UHI, which have practical implications for researching urban climate.

Measurements of turbulence transfer in the near-surface layer over the Antarctic sea-ice surface from April through November in 2016

AbstractThe surface energy budget over the Antarctic sea ice from 8 April 2016 through 26 November 2016 are presented. From April to October, Sensible heat flux (SH) and subsurface conductive heat flux (G) were the heat source of surface while latent heat flux (LE) and net radiation flux (Rn) were the heat sink of surface. Our results showed larger downwardSH(due to the warmer air in our site) and upwardLE(due to the drier air and higher wind speed in our site) compared with SHEBA data. However, the values ofSHin N-ICE2015 campaign, which located at a zone with stronger winds and more advection of heat in the Arctic, were comparable to our results under clear skies. The values of aerodynamic roughness length (z0m) and scalar roughness length for temperature (z0h), being 1.9 × 10−3m and 3.7 × 10−5m, were suggested in this study. It is found that snow melting might increasez0m. Our results also indicate that the value of log(z0h/z0m) was related to the stability of stratification. In addition, several representative parameterization schemes forz0hhave been tested and a couple of schemes were found to make a better performance.

Impact of Effective Roughness Length on Mesoscale Meteorological Simulations over Heterogeneous Land Surfaces in Taiwan

The Weather Research and Forecasting (WRF) modeling system obtains the aerodynamic roughness length (z0) from a land use (LU) lookup table. The effective aerodynamic roughness length (z0eff) was estimated for the island of Taiwan by considering the individual roughness lengths (z0i) of the underlying LU types within a modeling grid box. Two z0eff datasets were prepared: one using the z0i from the default LU lookup table and the other using the observed z0i for three LU types (urban, dry cropland and pasture, and irrigated cropland and pasture). The spatial variability of the z0eff distribution was higher than that of the LU table-based z0 distribution. Three WRF sensitivity experiments were performed: (1) dominant LU table-based z0 (namely, S1), (2) z0eff estimated from the default z0i (namely, S2), and (3) z0eff estimated from the observed z0i (namely, S3). Comparisons of the thermal field, temperature, and surface sensible and latent heat fluxes revealed no significant differences among the three simulations. The wind field overestimation and surface momentum flux underestimation in S1 were reduced in S2 and S3, and these improvements were more prominent over areas with highly heterogeneous land surface conditions.

Relationships between Sea Ice Concentration and Wind Speed over the Arctic Ocean during 1979–2015

AbstractNCEP CFSR reanalysis 6-hourly fields from 1979 to 2015 were used to investigate the relationships of sea ice concentration (SIC), atmospheric stratification, surface roughness, and wind speed at 10-m height (W10) and 850-hPa level (W850). We found that in autumn (September–November), winter (December–February), and spring (March–May) a lower SIC favors less-stable stratification and a higher W10. In autumn, the decrease in SIC is strongest, and SIC has its strongest correlation with the atmospheric stratification, W10, and the ratio of W10 and W850 (WSR). W10 and WSR have increased in autumn, and the negative trends in SIC typically are collocated with positive trends in W10 and WSR. In winter, W850 has negative trends over the Arctic Ocean, which, together with the lack of decrease of SIC in the central Arctic, has prevented W10 from increasing in winter. The winter trends are notably different from those for autumn, but the correlations are fairly similar. In autumn, winter, and spring, the negative correlation between SIC and W10 originated from the reduction of both stratification and aerodynamic surface roughness z0 with a reduction of SIC. The dependence of z0 on SIC is, however, weak in NCEP CFSR. In summer, the ratio of W10 and W850 has increased over large areas. The correlations between SIC and atmospheric variables were stronger on interannual time scales than on subseasonal time scales. The causal relationships are complicated by the two-way interaction between SIC and W10. In most cases, especially in summer, SIC increases after periods of W10 exceeding 5 m s−1.

A simplified method on estimation of forest roughness by use of aerial LIDAR data

AbstractIn the last decade, satellite‐based measurements combined with local land cover information have produced datasets with a very detailed land cover description. CORINE Land Cover (CLC) dataset is owned and maintained by the European Environmental Agency (EEA) and published at the agency's website. Another remote sensing tool, developed in the same period, is the terrain LIDAR scanners with very high resolution and porosity information. In the current study, LIDAR scans of mainland Denmark with 0.4 m resolution were used to estimate the aerodynamic roughness of large forests, the borders of which were defined with the help of the CLC dataset. The results are compared with available in situ measurement results from the scientific literature. There was a generally good agreement between calculated and measured displacement height values but less so for aerodynamic roughness values due to the employed spatial averaging process. The results reveal a promising application that can be used for forest parameterization within modeling tools.

Evaluating the impacts of updated aerodynamic roughness length in the WRF/Chem model over Pearl River Delta

The aerodynamic roughness length (Z0) plays an important role in the land–air exchange. At present, the atmospheric numerical models cannot accurately describe the Z0 in different underlying surfaces, which affects the simulation performance and the description of the momentum and heat exchange and atmospheric environment. In this study, the regional air quality model WRF/Chem with updated Z0 of the various underlying surface categories is used to study the impacts of Z0 on the boundary layer meteorological conditions and atmospheric environment over the Pearl River Delta (PRD) region. Two numerical experiments are conducted, including default Z0 (Base experiment) and updated Z0 (Case experiment). By comparing the simulations with the observations from 9 meteorological stations and 30 national air quality monitoring stations, the updated Z0 improves the model simulation performance of 10 m wind speed in PRD. The impacts of updated Z0 on 2 m temperature and relative humidity are relatively small, while the influences of updated Z0 on surface skin temperature (TSK), ground heat flux (GRDFLX), sensible heat flux (HFX), friction velocity (U*), 10 m wind speed (WS) and planetary boundary layer height (PBLH) are evident. The impacts of updated Z0 in the urban are greater than that in the evergreen broadleaf forest. In the urban, the updated Z0 reduces TSK, lowers GRDFLX, increases HFX during the daytime, and increases U*, causing the reduction of WS, while the vertical velocity also increases, which is conducive to the elevation of PBLH. With the updated Z0, the surface carbon monoxide (CO) concentrations generally increase, especially in the central urban areas, due to the impact of wind speed, turbulent motion and planetary boundary layer height. The updated Z0 has a little impact on CO concentration profile during the daytime, which is affected by horizontal wind speed and turbulent motion; while at night, the updated Z0 produces an increase of CO concentration below 300 m, which is affected by the horizontal wind speed.

Parametrizing Horizontally-Averaged Wind and Temperature Profiles in the Urban Roughness Sublayer

Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin–Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment.

Nonlinear boundaries of land surface temperature–vegetation index space to estimate water deficit index and evaporation fraction

Land surface temperature (Ts)–vegetation index (VI) space is widely used for agricultural drought monitoring. The determination of dry and wet edges is the essential procedure for the construction of this space. However, the universally applied linear boundaries have not been strictly verified. In this study, we aim to: (1) develop and analyze the nonlinear boundaries of Ts–VI space using one-source surface energy balance theory, and (2) validate the improvement of nonlinear boundaries with in-situ measurements in the view of water deficit index (WDI) and evaporation fraction (EF). Results demonstrate the nonlinear characteristics of dry and wet edges and show that the nonlinear boundaries are the single-valley functions of fractional vegetation cover (fc), sharply decreasing with fc in sparse canopies and slowly increasing with fc in dense canopies. The nonlinear feature is controlled by aerodynamic and thermodynamic roughness lengths, which reflect the coupling effect between soil and canopy. In comparison with linear boundaries, nonlinear boundaries lead to significant improvement on estimating WDI but similar performance on estimating EF. The rough estimation of EF using nonlinear boundaries may be attributed to the high uncertainty of aerodynamic resistance. Our analysis suggests that the uncertainty of surface roughness length schemes and energy imbalance issue are the main error sources of nonlinear dry and wet boundaries.

Dynamic response of land–atmosphere-coupling parameters to precipitation in the sparse-vegetated Asian summer monsoon transition zone

Land–atmosphere-coupling parameters usually relate to particular climatic conditions and seldom consider the influence of the inter-annual variability of precipitation. This results in large uncertainty in the estimation of land-surface physical variables. In the current study, observed data for five land–atmosphere-coupling parameters (surface albedo, soil thermal conductivity, aerodynamic roughness length, and the bulk transfer coefficients of momentum and sensible heat) were analysed and related to the inter-annual variability of precipitation on the Loess Plateau, China, from April 2006 to March 2013. This is an area with sparse vegetation. The results demonstrate that the land–atmosphere-coupling parameters are very sensitive to the inter-annual variability of precipitation. The surface albedo increased with increasing duration of snow cover. The other four parameters increased when annual effective precipitation (yearly sum of daily rainfall > 4 mm or daily snowfall > 0.1 mm) increased, and the sensitivity decreased when annual effective precipitation increased. Empirical relations between the five land–atmosphere-coupling parameters and the inter-annual variability of precipitation were derived via regression, with R2 values of 0.67, 0.85, 0.93, 0.99, and 0.95, respectively. The land-surface physical variables calculated with dynamic land–atmosphere-coupling parameters (considering the inter-annual variability of precipitation) were much closer to the observed data than those calculated with static land–atmosphere-coupling parameters. The relative error was reduced by > 80% in years with low precipitation. This indicates that the inter-annual variability of precipitation has a significant impact on the land-surface physical variables. The results demonstrate that land–atmosphere-coupling parameters, which take into account inter-annual variability of precipitation, provide a more realistic representation of the land surface.

Non-Equilibrium Scaling Applied to the Wake Evolution of a Model Scale Wind Turbine

The present paper addresses the evolution of turbulence characteristics in wind turbine wakes immersed in a turbulent boundary layer. The study thereby focuses on finding physically consistent scaling laws for the wake width, the velocity deficit, and the Reynolds stresses in the far wake region. For this purpose, the concept of an added wake is derived which allows to analyse the self-similarity of the added flow quantities and the applicability of the non-equilibrium dissipation theory. The investigation is based on wind tunnel measurements in the wake of a three-bladed horizontal axis wind turbine model (HAWT) immersed in two neutrally-stratified turbulent boundary layers of different aerodynamic roughness length. The dataset also includes wake measurements for various yaw angles. A high degree of self-similarity is found in the lateral profiles of the velocity deficit and of the added Reynolds stress components. It is shown that these can be described by combined Gaussian shape functions. In the vertical, self-similarity can just be shown in the upper part of the wake. Moreover, it is observed that the degree of self-similarity is affected by the ground roughness. Results suggest an approximately constant anisotropy of the added turbulent stresses in the far wake, and the axial scaling of the added Reynolds stress components is found to be in accordance with non-equilibrium dissipation theory. It predicts a x − 1 decay of the added turbulent intensity I + , and a x − 2 evolution of the added Reynolds shear stresses Δ u i ′ u j ′ ¯ and the velocity deficit Δ u . Based on these findingsa semi-empirical model is proposed for predicting the Reynolds stresses in the far wake region which can easily be coupled with existing analytical wake models. The proposed model is found to be in good agreement with the measurement results.